Yesterday morning, in the early sunshine, there was an extra decoration on Heath Street. Lying on the pavement, six inches from the road, was a crumpled pair of woman's knickers - mauve lace with chocolate-brown borders. From the position, I would imagine that they had been thrown there from a passing car sometime the previous night.

You can decide for yourself what to make of this inappropriate disposal of lingerie. My first reaction was negative. I imagined some incontinent couple, first meeting that evening in a nightclub, engaging in some ambulant, semi-public sex.

Is this just a prejudice? Things, many people think (and not just church-people), have gone too far. It was all very well to lift some of the most irrational taboos on sexual activity, but hasn't the consequence of liberalisation been a coarsening of personal life, a cheapening of sex, a removal of the necessary restraint on an animal impulse? We now have The Lord of the Trouser Flies. And isn't this, an increasingly loud public voice demands, partly the result of abused, indiscriminate knowledge?

When the biopic Kinsey was recently released in the US, there was a controversy that probably wouldn't have happened 10 years ago. Objectors were widely reported as criticising the director, Bill Condon, for being a gay-rights activist determined to show the influential sex researcher in a positive light. As one academic put it: 'The main source of the sexual scandal today is Dr Kinsey's sexual revolution.' But that 'sexual revolution' was not a consequence of Kinsey's doctrines or teachings;rather, it sprang out of what he claimed to have discovered, through meticulous research, about human sexual behaviour. Kinsey's revolution was about knowledge.

I watched Kinsey last week. And, by a coincidence, it was the same week in which the leader of Scotland's Roman Catholics, Cardinal Keith O'Brien, called for a boycott of the National Lottery because of the money given by the lottery to the Brook Advisory Centres and to the FPA, formerly the Family Planning Association. Both organisations give help and advice to pregnant women seeking abortions, as well as providing sex-education material to schools, parents and youth organisations.

The cardinal, however, linked them with a doomed strategy. 'As far as I am aware, rates of teenage conceptions, sexually transmitted infections and abortions continue to rise,' he said. 'In short, we are left facing the single, incontrovertible fact that all our current approaches have failed. Yet we continue to fund on an enormous scale the very work which has so demonstrably failed.'

It so happens that 2005 sees the 75th anniversary of the FPA. Set up as the National Birth Control Council in 1930, the organisation started with one small room and one worker. Its mission was to help 'married people [to] space or limit their families and thus mitigate the evils of ill-health and poverty'. In that year, it persuaded the Lambeth conference of Anglican bishops, by 193 votes to 67, to approve of the new body and its policies, on the basis that Christians could accept a compelling argument for birth control 'where there is a morally sound reason for avoiding complete abstinence'.

Since then, the FPA, on whose executive I sit, has been providing non-judgmental advice, information and help on sex and pregnancy to people of all ages. This is the approach that the cardinal argues has failed.

And Kinsey does have something to do with it. The American academic was responsible for the most significant research ever undertaken on sexual behaviour. The Kinsey Report consisted of an analysis of many thousands of elaborately taken 'histories' from Americans of all ages and all backgrounds. Brought together in 1948 and in 1953 with the publications of Sexual Behavior in the Human Male and Sexual Behavior in the Human Female, the research transformed our knowledge of what people actually did.

It showed for the first time, for example, that just about all men masturbated. It suggested that a third of men had, at one time or another, had homosexual experiences. And just as they indicated that there were some activities which all or many people engaged in, Kinsey also revealed that there was a wide variation in levels and types of sexual activity.

For the boy tortured by the belief that his masturbation was uniquely sinful, this knowledge represented the lifting of a huge burden. For the woman attracted to other women, but who had never heard the word 'lesbian', let alone (she thought) met one, it was a revelation. Before Kinsey, much sex in the West was accompanied by a guilty misery or by the threat of persecution. People died, killed themselves or were killed, because of sexual ignorance and prejudice.

Yet this ignorance was preferred by some. One New York pastor described Kinsey as 'a deranged Nebuchadnezzar', luring women 'out into the fields to mingle with the cattle and become one with the beasts of the jungle'. Evangelist Billy Graham condemned him for opening the Pandora's box of sexual knowledge. That was 50 years ago, so why am I writing this? We can never unknow what we know about wanking and copulation.

But we can still make a terrible botch of the way we teach sex and the way we describe it. Look at Scotland: it has one of the highest rates of teenage pregnancy in Europe; gonorrhoea and chlamydia rates have risen, the latter by 39 per cent between 1999 and 2002. In 1999, the Scottish Health Minister promised a sexual-health strategy, which it took until January of this year to get published.

When, in January, the strategy finally materialised, it was bizarre. Described by Labour Health Minister Andy Kerr as 'Abstinence Plus', it started from the premise that young people shouldn't have sex, but that if they were going to, then it should be safe. This reminds me of the rules in Balliol College, Oxford, in the early 1970s. Rule one stated that no women visitors were to stay overnight; rule two said that no women visitors were to stay for breakfast.

And though the Scottish strategy laid down a programme of sex education in schools, it exempted faith schools from the access to advice and services available to other pupils. And even in state schools, parents would have the right to pull their children out of sex-education classes. Said the hyperactive but happy Cardinal O'Brien: 'I am pleased the Health Minister recognises marriage as "a key pillar of our national life" and that he endorses the principle of abstinence.'

My ignoble first reaction was to want to write to the cardinal telling him how much I hoped that my daughters, always given the information that they need, did not go out with ignorant Catholic boys. Before anger, however, we require understanding, because something has gone wrong. We do, as the critics argue, have a sexualisation of culture which can be very damaging.

But this is not, I would argue, to do with knowledge. In fact, it's the reverse. You are allowed to be stupid, prurient and exploitative about sex. You just aren't allowed to be honest. You can have adverts full of dodgy attitudes and competitiveness, but you can't have proper, compulsory sex education in schools. Cardinal O'Brien wanted 'no graphic images' in sex instruction. Sex without penises, without vulvas, without bodies.

Today, we see sex as athletics, sex as marketing, and, courtesy of the cardinal and others, sex as a form of pathology, something to be avoided for fear of sin or disease. This is why our girls get pregnant too early, our boys get the clap, why they know so little about what they spend so much time wanting and fearing.

Rarely, though, do they hear us discuss sex as enjoyable, great, something positive. Sex aversion of one sort or another still rules. Perhaps it always will until, somewhere in between hockey and citizenship, there are masturbation classes.

That's a joke, of course, but it is worth reiterating that knowledge is better than ignorance and free choice is better than suppression, and we have never, despite the efforts of Kinsey and the FPA, had enough of either. So I salute the first, wish happy birthday to the second and admit that, in all probability, the owner of the chocolate and mauve knickers may have been having rather a nice evening.